In his strongest attack on any union leader, Miliband tore into Len McCluskey hours after the Unite general secretary claimed that the Labour leader would be "cast into the dustbin of history" unless he abandons support for David Miliband's campaign managers, Jim Murphy and Douglas Alexander.

"Len McCluskey does not speak for the Labour Party," a spokesman for Miliband said. "This attempt to divide the Labour Party is reprehensible.

"It is the kind of politics that lost Labour many elections in the 1980s. It won't work. It is wrong. It is disloyal to the party he claims to represent."

The strongly worded statement follows a warning by a member of the shadow cabinet that Britain's largest trade unions have been taken over by a new "Bennite tendency" which must be fought by Labour. McCluskey has been identified as one of the most disruptive forces.

Miliband, who has been accused by the Tories of being in the pay of unions after they secured him the Labour leadership, felt he had no choice but to attack McCluskey after his highly personal attack on two senior members of the shadow cabinet. The Labour leader was also furious when McCluskey used an interview in the New Statesman to hint that Unite may take unilateral action with other unions to call a general strike in the face of opposition in the TUC.

McCluskey told the New Statesman: "Ed Miliband must spend most of his waking hours grappling with what lies before him. If he is brave enough to go for something radical, he'll be the next prime minister. If he gets seduced by the Jim Murphys [shadow defence secretary] and the Douglas Alexanders [shadow foreign secretary], then the truth is that he'll be defeated and he'll be cast into the dustbin of history."

The Unite general secretary, who also criticised the shadow work and pensions secretary, Liam Byrne, said Miliband must offer a radical alternative to the programme of cuts, to prevent David Cameron running a "stick with me" campaign. "My personal fear, and that of my union, is that if he goes to the electorate with an 'austerity-lite' programme then he will get defeated," he said. "I'm fairly confident that Cameron will go to the electorate in two years' time and basically his message will be: 'Stick with me.'

"I've just observed Barack Obama being elected … where there was a very similar message that he put out to the American people, he repeated over and over again, stick with me. And they did do. And so my fear is that if Ed is simply offering the British electorate an austerity-lite programme, that won't capture their imagination."

McCluskey spoke out after Tony Blair warned in the recent centenary edition of the New Statesman that Labour must avoid returning to its "comfort zone" in which it attacks government cuts without spelling out a credible alternative. The Unite general secretary said: "My message to Ed is to take no notice of the siren voices from the boardrooms of JP Morgan or wherever else he [Blair] is at the moment … It may be easy for these people, who are sitting with the huge sums of money that they've amassed now – they've done pretty well out of it … But the fact is that under Labour the gap between rich and poor increased."

"The truth is that this is a process that was set up by Tony Blair, and the rightwing and organisations like Progress have had it their own way for years and years and have seen nothing wrong in it," McCluskey said.

"Because we're having some success, suddenly these people are crying foul. Well I'm delighted to read it. I'm delighted when Blair and everyone else intervenes because it demonstrates that we are having an impact and an influence and we'll continue to do so."

McCluskey qualified his remarks by saying that Miliband has done a "good job" since his election as leader in 2010. David Miliband won most votes among Labour MPs and MEPs and Labour party members. But Ed Miliband prevailed after the main union leaders agreed to back him.

Murphy tweeted: "It's disappointing in advance of important local elections that Len McCluskey turns his fire on Labour."

McCluskey also made clear he was spoiling for a fight on a general strike. "I very much doubt the TUC will name a day because I think it's true that the majority of unions are not in favour of such a call," he said. "But some unions, including Unite, might go away and talk among themselves about whether there is anything else they might wish to do, over and above the collective decision of the TUC."